The invention is related to storage systems and in particular to snapshot methods in storage systems.
A method for creating a snapshot volume (the volume where the data image at a certain point-in-time is preserved) is frequently used in many storage systems, since the snapshot enables users to backup data on a live system concurrently when host I/O is online. Generally, there are two methods for taking snapshots:
Mirroring method: In this method, the storage system physically creates a mirror of the volume (primary volume, production volume) which constitutes the snapshot. The snapshot is created on a secondary volume (snapshot volume). When the user instructs the storage system to stop mirroring, a point-in-time image of the data in the primary volume is preserved in the secondary volume. An example of this method is disclosed in U.S. Pat. No. 6,092,066 and is incorporated herein by reference in its entirety.
Copy-on-write method: In the mirroring method described above, users have to use a secondary volume having the same size as the primary volume to store the snapshot. This doubles the storage cost for maintaining snapshots. In the copy-on-write method, the storage system does not mirror all of the data that is stored on the primary volume. Instead, when updates occur to a region in the primary storage systems, the storage system copies the data in the region (hereinafter it is called “differential data”) to a preservation memory area (e.g., magnetic disks, semiconductor memory, or others). Thus, the data is written to both the primary volume and to the preservation memory area. In this method, the amount of preservation memory area usually is much less than the size of the primary volume since only updated regions in the primary volume are duplicated and it is rare that all of the primary volume would be re-written between snapshot volumes. U.S. Pat. No. 5,649,152 discloses the copy-on-write snapshot method and is incorporated herein by reference in its entirety.